Title: Extend PerfAnnotationContext with begin and end events.

Description:
The current interface of PerfAnnotationContext allows easy recording of durations for a region of code inside a single lexical scope with PERF_OPERATION and PERF_RECORD annotations. But sometimes it might be useful to record durations between events happening in different scopes, e.g. how long it takes to destroy an ActiveTcpConnection after its corresponding ActiveStream has been destroyed (given they are 1-to-1).

I'd like to propose two additional annotations PERF_BEGIN(category, description) and PERF_END(category, description) asserting that a closing PERF_END always happens after PERF_BEGIN.